1. Field of the Invention
The present invention generally relates to a conveying device conveying a recording medium, such as a recording sheet, on which an image is printed by jetting a liquid such as an ink thereto, a conveyance control device, and an inkjet recording device printing such an image on the recording medium, and more particularly, to, a conveying device which can convey the recording medium with high precision so as to increase a positional precision of applying the liquid onto the recording medium to stably form a high-quality image on the recording medium.
2. Description of the Related Art
A full-colored image can be formed by an electrophotographic method including steps of forming an electrostatic latent image on a photosensitive member, developing each color, and then overlapping the developed colors. However, in performing this method, the step of overlapping the colors is difficult. Further, the method involves complicated component devices including developing devices around the photosensitive member, which enlarges an image-forming device as a whole, and increases costs thereof. By contrast, with an inkjet recording device printing on a recording sheet by jetting ink drops thereon, it is easy to overlap the colors. Additionally, the inkjet recording device has a small recording head jetting the ink drops, which reduces the size of the image-forming device as a whole. Also, the inkjet recording device is excellent in terms of a photographic quality. Further, an image formed on an OHP by the inkjet recording device has an excellent optical transmittance.
In pursuit of a higher-quality image in this inkjet recording device, the ink drops need to be jetted to landing spots on the recording sheet with higher precision. Therefor, not only the recording head jetting the ink drops needs to be further sophisticated in structure, but also the recording sheet needs to be conveyed with higher precision. In a serial printer of an inkjet type, the recording sheet is stopped while the recording head performs a scanning. Accordingly, the recording sheet is repeatedly conveyed and stopped. At this point, a precision of conveying the recording sheet means conveying the recording sheet a predetermined distance, and thereafter stopping the recording sheet at a predetermined position.
For the purpose of enhancing the precision of conveying the recording sheet, in inkjet recording devices disclosed in Japanese Laid-Open Patent Application No. 4-201469, Japanese Laid-Open Patent Application No. 9-254460, and Japanese Laid-Open Patent Application No. 2000-25249, for example, a conveying belt conveying the recording sheet is uniformly charged positively so that the recording sheet is stick fast to the conveying belt by an electrostatic force so as to prevent the recording sheet from being displaced. However, in this state, the ink drops jetted from the recording head are influenced by an electric field such that landing spots of the ink drops are displaced on the recording sheet. For the purpose of preventing this displacement of the landing spots of the ink drops, a negative charge is applied to the conveying belt uniformly charged positively on the surface, in the vicinity of the recording head so as to lessen the electrostatic force so that the ink drops jetted from the recording head are not influenced by the electric field, as described in Japanese Laid-Open Patent Application No. 2000-25249, for example. Additionally, as described in Japanese Laid-Open Patent Application No. 4-201469, for example, a circumferential groove is formed at a predetermined position in an axial direction of a conveying roller, and a projection is formed at a position in a widthwise direction of the conveying belt wound around the conveying roller, the position corresponding to the circumferential groove of the conveying roller, wherein the projection of the conveying belt is engaged in the circumferential groove of the conveying roller so as to regulate the widthwise position of the conveying belt.
However, applying the negative charge, as mentioned above, to the conveying belt in the vicinity of the recording head so as to lessen the electrostatic force necessitates a means therefor, which disadvantageously complicates an entire structure of a conveying device conveying the recording sheet. Additionally, after an image is recorded on the recording sheet by jetting the ink drops thereon, separating the recording sheet from the conveying belt necessitates a separating force that exceeds the electrostatic force applied throughout the surface of the recording sheet, which makes it difficult to provide a separating unit capable of separating the recording sheet. Further, when the conveying belt slips on the conveying roller, the recording sheet is displaced in a direction in which the recording sheet is conveyed by the conveying belt, reducing the precision of conveying the recording sheet.
Additionally, when an image is recorded on the recording sheet by jetting the ink drops thereon, the recording sheet is elongated due to water contained in the ink drops. This phenomenon is referred to as a cockling. Due to this cockling, the recording sheet becomes wavery such that the distance between nozzles of the recording head and the surface of the recording sheet varies depending on the position. When this, cockling becomes aggravated, the recording sheet contacts the nozzles of the recording head, at worst, such that the nozzles of the recording head become dirty, and that the recording sheet is smeared. Further, this cockling may displace the landing spots of the ink drops on the recording sheet. In order to prevent these, influences of the cockling, an image is formed on a recessed platen by jetting the ink drops from the recording head thereon, while the recording sheet is pressed by a spur having projections on its periphery. However, pressing the recording sheet by the spur may leave a scar on the image formed on the recording sheet, resulting in a deterioration of the image. Also, in order to prevent the above-mentioned influences of the cockling, there is another measure as described in Japanese Laid-Open Patent Application No. 2000-191175. According to this measure, protruding parts and recessing parts are formed on the surface of the conveying belt. The recessing parts are provided with air inflow ports. Vacuum suction is performed via these air inflow ports so as to vacuum-suck the recording sheet to the conveying belt. Thereby, the waves occurring in the recording sheet undergoing the above-mentioned cockling are made lower such that the recording sheet does not contact the recording head.
However, when the recording sheet is vacuum-sucked by the recessing parts of the conveying belt so as to prevent the above-mentioned influences of the cockling of the recording sheet, the recording sheet in the vicinity of the recording head also becomes uneven according to the protruding parts and recessing parts formed on the surface of the conveying belt. This unevenness on the recording sheet displaces the landing spots of the ink drops on the recording sheet, which results in a deteriorated image.
Further, there is also an inkjet recording device comprising a pair of conveying rollers, one of the pair being a combination of the above-mentioned spur and a roller, in which the pair of the conveying rollers convey the recording sheet. In this device, the precision of conveying the recording sheet can be guaranteed only when the recording sheet engages the pair of the conveying rollers. In recent years, there has been a need for an expansion of an image printing area. For the purpose of securing this printing area, there is also an inkjet recording device forming an image in a state that cannot essentially guarantee the precision of conveying the recording sheet, i.e., in a state where the recording sheet engages only either of the pair of the conveying rollers. When an elevation of the recording sheet occurs in this state, the inkjet recording device is unable to deal with this elevation, and cannot secure a force for conveying the recording sheet; thereby, the precision of conveying the recording sheet cannot be guaranteed, and a quality of an image is reduced.